We plan to initiate a long term study of the specific chemical interaction which take place when a steroid molecule binds its receptor protein. This basically includes the identification of functional groups present in the binding site and the examination of conformational changes which appear to take place in the protein when binding occurs. The approach is to use the well described methods of protein chemistry to examine the estradiol binding protein(s) of the Beta-globulin fraction of human plasma. We are attempting through these studies to focus our attention on the initial step involved in the mechanism of steroid hormone action i.e., the binding to the receptor protein molecule. The specific research problems to be undertaken are as follows: 1. The purification of the estradiol binding protein(s) of human plasma. Development of preparative methods to isolate significant amounts of the protein(s) for further study. 2. Study of the reversible association of the (estradiol-protein) complex. Isolation of the apoprotein. 3. Physical and chemical characterization of the binding protein and the apoprotein. 4. Chemical modification and active site labeling of the estradiol binding protein to gain further insight on the type of amino acid residues important (1) in the binding mechanism and (2) in the maintenance of the active conformation. 5. Influence of the purified Beta-globulin on the incorporation of estradiol in uterine cells (in vitro experiments). These results will hopefully provide preliminary information on the nature of the steroid binding phenomenon and on the general features of the protein molecule involved in this process. Furthermore, the studies may establish a specific fundamental role of the plasma Beta-globulin in the mechanism of action of estradiol.